The present invention relates to a method for manufacturing a recording medium that has a layer structure with at least one layer of a signal recording layer on the surface of a support substrate comprising essentially of a thermoplastic resin. Described in more detail, the present invention relates to a recording medium manufacturing method having a surface treatment process that removes contamination on the surface of a thermoplastic resin support substrate and also improves its wettability. In addition, this surface treatment process generates little deterioration in the support substrate surface.
A hard disk, which is used as the main memory device for a computer, uses Al and glass and the like for a support substrate, and a layer structure, including a recording layer, is formed on this substrate surface. In general, the hard disk has a construction in which the recording layer is exposed at the surface. In addition, the head, which conducts recording and playback, is a non-contact type, but the head conducts recording and playback while moving at a height of 50 nm or less from the hard disk surface. Therefore, the smallest defects and contamination on the hard disk are problematic. Contamination on the hard disk can be due to peeling of the recording layer resulting from contact between the recording layer and the head, and it can be due to polishing dust resulting from the processing of the support substrate. In order to prevent the generation of this type of contamination, various attempts have been made to improve the adhesion between the recording layer and the support substrate. In addition, in order to remove the polishing dust, rinsing is conducted with a sponge and the like using pure water and surface active agents and the like.
When rinsing using a mechanical force in this way, the rinsing is inadequate particularly at the end surfaces of the substrate. Therefore, there has been developed a substrate rinsing method (Japanese Laid-Open Patent Number 5-143981) that has a process of placing the Al substrate in an ozone atmosphere and, at the same time, irradiating this substrate with ultraviolet light.
With advances in the development of resins with excellent shape stability, substrates that are easier to mold and are inexpensive compared to Al or glass can be manufactured. For this reason, there have been attempts to use substrates of an injection molded thermoplastic resin such as polycarbonate resin and polyolefin and the like as the support substrates for hard disks. Support substrates comprising essentially from thermoplastic resins generally have a low surface wettability. As a result, when a layer structure, which includes a recording layer, is formed on the substrate surface, the adhesion is poor, and peeling of the recording layer can easily occur due to contact between the recording layer and the head. Recording media that use substrates comprising of thermoplastic resin include magneto-optical disks, compact disks, and the like. However, with these recording media, they have structures in which the recording layer is not exposed at the surface. As a result, recording and playback is conducted through non-contact. Therefore, compared to hard disks in which the recording layer is exposed, contamination resulting from the peeling of the recording layer is not such a problem. However, with hard disks, this type of contamination must be eliminated.
However, compared to the traditional substrates of Al and glass, support substrates of thermoplastic resin are more easily damaged and absorb water more readily. As a result, the method of removing contamination on the substrate surface by rinsing with a sponge and the like using pure water and surface active agents and the like is not suitable. In addition, in Japanese Laid-Open Patent Publication 5-143981, there is simultaneous action of ozone and ultraviolet light. However, for support substrates made of thermoplastic resin, when using a rinsing method which does not have a process for aggressively stopping the action of ozone and ultraviolet light, not only will the surface contamination be removed, but there can be degradation of the substrate itself.